(1) Field of the Invention
The present invention relates to the field of nuclear medicine instrumentation. Specifically, the present invention relates to apparatus and methods for processing signals within a gamma camera system.
(2) Prior Art
Gamma camera systems contain one or more scintillation detectors each coupled to signal processing hardware and to an image generation system that may be one in the same. The scintillation detector is composed of an array of photomultiplier tubes (PMTs) each generating a individual channel signal that is responsive to light energy emitted by scintillations from within a crystal layer that is optically coupled to the PMT array. The scintillations are responsive to interactions between the crystal and a gamma event. The signal processing hardware typically processes the channel signal including multiplying the signal by a preamplification gain and also integrating the channel signal to arrive at an integrated result of an energy value.
The channel signals from a group of photomultipliers are then analyzed in a mathematical procedure to determine a spatial coordinate of the event in two dimensional space (X, Y). The process to perform the spatial computation utilizes a group or cluster of PMTs that surround a central PMT near which a gamma event occurred. In a centroiding procedure, the coordinate of the gamma event is determined by analyzing the energy signals of each PMT of the cluster and the total energy of the gamma event (as reported from all PMTs) using a weighted average approach. In the prior art, the constitution and geometry of this cluster was fixed for all central PMTs. In other methods of the prior art, the cluster was defined by all PMTs having an energy signal over a predetermined threshold amount. Again cluster is fixed by the threshold amount.
However, because of particular optical and geometric characteristics of the scintillation detector, use of a fixed geometric cluster is not advantageous. For instance, a fixed geometric cluster does not offer variations in the resolution for a gamma event coordinate computation. Also, for central PMTs that lie on the edge or corner of a scintillation detector, the fixed geometric cluster is often asymmetrical about the central PMT and this can lead to inaccurate spatial computations. Rather, what is needed is a gamma camera system that can define a unique cluster constitution (e.g., geometry and number of PMTs included) for each central PMT (e.g., the PMT having the largest energy response to the gamma event). The present invention provides such advantageous results.
Within the centroid computation procedure used to determine the spatial coordinate of a gamma event, a weighted average procedure is utilized based on weight factors assigned to each PMT for the X and Y coordinates. In the prior art systems, the weights (X and Y) assigned to each PMT are fixed values. However, the PMT contribution to the centroid computation is not always a fixed factor. Other factors, such as the crystal boundaries, optical interfaces, and PMT photocathode properties can make the PMT contribution different depending on the location of the gamma event and the position of the PMT with respect to the overall PMT cluster configuration. What is needed is a system that can provide variable X and Y weights to PMTs to permit higher accuracy centroiding and reduce demands on correction processing stages and also for permitting a larger field of view of the camera system without increasing the crystal dimensions. The present invention offers such advantageous functionality.
Accordingly, it is an object of the present invention to provide better image determination within a gamma camera system. It is yet another object of the present invention to provide spatially variant cluster constitution (configuration) to provide variable resolution within the gamma camera system and to provide more accurate determination of the spatial coordinates of gamma events. It is another object of the present invention to provide spatially variant weight assignments for PMTs within a given cluster to more accurately determine the spatial coordinates of gamma events. It is further an object of the present invention to determine the above variable weight assignment based on a particular PMT's location and also based on a PMT cluster type. These and other objects of the present invention not specifically mentioned above will become clear within discussions of the present invention herein.